Image display apparatus and head-up display system

ABSTRACT

An image display apparatus includes an image forming element, an illumination optical system that guides light emitted from a light source to the image forming element, and a plurality of lenses that project a trapezoidal image formed by the image forming element onto a projected surface. The plurality of lenses have an optical axis that is at an inclination of 1° or more with respect to a perpendicular of a plane including an image forming surface of the image forming element. First rays of the light have a first focal point and second rays of the light have a second focal point. The first focal point is farther from the image forming element than the second focal point is from the image forming element. The first rays form the first portion of the trapezoidal shape and the second rays form the second portion of the trapezoidal shape.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/637,989, filed Jun. 29, 2017, which claims priority to JapanesePatent Application No. 2016-129037, filed in Japan on Jun. 29, 2016,contents of which are incorporated herein by by reference.

BACKGROUND

An image forming device generally is commonly referred to as an imagedisplay device or a “projector,” and such projectors may include acathode ray tube (CRT) projector, a liquid crystal projector, or a DMD(Digital Micro mirror Device) projector. A DMD is a reflection typeimage forming device or image display device. A DMD includes a pluralityof micromirrors arranged in a two dimensional array that turns thereflected light on and off by changing a tilt angle of each individualmicromirror.

A projector has been used for information sharing at office and schooland watching movies and other images. Recently, it is used for digitalsignage that displays advertisements and guidance on electronic mediaand is also used in a head-up display (HUD) system which projects animage on a small screen arranged in a car. Such a head-up display systemcauses the driver to observe the image via a concave mirror and awindshield of the car.

However, a projector used for digital signage has a problem that animage projected on a projected surface becomes trapezoidal and a focusof the projector becomes unbalanced at the top and bottom of the imagewhen the projected surface is inclined with respect to the optical axisin the case of oblique projection. When focusing on the bottom edge ofthe image, the top edge of the image will not be in focus, so a methodto reduce the focus imbalance by focusing on the center of the image isused.

In a projector for an automobile HUD, it is sometimes preferable thatthe image projected on the small screen has a trapezoidal shape. Theimage to be projected is, for example, a speedometer, a road sign, orthe like, and the unbalance of focus of the image is required to beminimal.

As a method for eliminating the imbalance in focus of the image, amethod of adjusting the optical system to satisfy the Scheimpflugcondition may be used. For example, by tilting the image plane (imageforming element) in accordance with the inclination of the object plane(projected plane), it is possible to focus on the entire area to beprojected. The amount of tilting the image forming element isproportional to the magnification of the lens system.

On the other hand, in the design of the illumination optical system ofthe general projector, it is required to uniformly illuminate the imageforming element. For example, an optical system that illuminates animage forming device using a light tunnel, and shows a configuration ofan illumination optical system for forming a real image at the exit ofthe light tunnel on the image forming device.

A configuration using a fly's eye lens may also be used. A system inwhich light from a light source is once divided into a plurality oflight fluxes by a fly's eye lens and is converted into a single bundleof rays again on the image forming element, thereby achieves uniformilluminance. Also in this case, the lens between the fly's eye lens andthe image forming element is designed so as to be in focus on the entireimage forming surface of the image forming element, and the imageforming element is uniformly illuminated.

As a light mixing element, the method of making the light amount uniformis different between a light tunnel and a fly-array lens, but in eithercase, there is a condensing optical system composed of a lens or amirror between the light mixing element and the image forming element.As described above, in a general projector, it is proposed that atechnique of projecting an image having a uniform illuminancedistribution onto the projected surface by making the illuminancedistribution on the image forming element uniform.

However, in a case where the projected surface is inclined with respectto the optical axis of the projection optical system and the image to beintentionally projected is trapezoidal (for example, digital signage orprojection in a HUD system for automobiles), if the illuminance on theimage forming element is uniform, the illuminance distribution of theprojected image becomes unbalanced.

Since the illuminance is a value obtained by dividing the light quantityby the area, the illuminance on the short side of the trapezoidal shapeimage is high and the illuminance on the long side is low, so that forthe observer, the short side is bright and the long side It will bevisually recognized as a dark unbalanced image.

Accordingly, an object of the present application is to provide an imagedisplay device capable of eliminating an imbalance in illuminancedistribution caused by a trapezoidal shape when the projected image is atrapezoidal shape.

SUMMARY

An image display apparatus in accordance with the present applicationcomprises a light source, an image forming element on which a pluralityof micromirrors are arranged, an illumination optical system for guidinglight emitted from the light source to the image forming element, and aprojection optical system having a refractive optical system including aplurality of lenses sharing an optical axis and projecting an imageformed by the image forming element onto a projected surface.

The optical axis of the refractive optical system has an inclination of1° or more with respect to a perpendicular of a plane including an imageforming surface of the image forming element. The image projected on theprojected surface is a trapezoidal shape that includes a first portionand a second portion, the first portion is parallel to the secondportion and the second portion is longer than the first portion. Theillumination optical system includes a condenser lens and a field lensas a condensing optical system, the condenser lens is between the fieldlens and the light source. A focal point of the condensing opticalsystem is closer to the field lens than to the image forming surface ofthe image forming element.

First rays of the light have a first focal point and interact with afirst side of the image forming element. Second rays of the light have asecond focal point and interact with a second side of the image formingelement. The first focal point is farther from the image forming elementthan the second focal point is from the image forming element. The firstrays form the first portion of the trapezoidal shape and the second raysform the second portion of the trapezoidal shape.

A head-up display system in accordance with the present applicationcomprises an image forming element, an illumination optical system thatguides light emitted from a light source to the image forming element, aplurality of lenses that project a trapezoidal image formed by the imageforming element, and an observation imaging optical system for forming avirtual image based on the trapezoidal image. The observation opticalsystem includes a concave mirror for enlarging and projecting thetrapezoidal image toward a transmissive reflective member.

The plurality of lenses have an optical axis that is at an inclinationof 1° or more with respect to a perpendicular of a plane including animage forming surface of the image forming element. The trapezoidalimage includes a first portion and a second portion, the first portionis parallel to the second portion and the second portion is longer thanthe first portion. The illumination optical system includes a condenserlens and a field lens, the condenser lens is between the field lens andthe light source. First rays of the light have a first focal point andinteract with a first side of the image forming element. Second rays ofthe light have a second focal point and interact with a second side ofthe image forming element.

The first focal point and the second focal point are closer to the fieldlens than to the image forming surface of the image forming element. Thefirst focal point is farther from the image forming element than thesecond focal point is from the image forming element. The first raysform the first portion of the trapezoidal shape and the second rays formthe second portion of the trapezoidal shape.

The above and other objects, features, advantages and technical andindustrial significance of this application will be better understood bythe following detailed description. A when considered in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing a general configuration of aprojector including a reflective image forming device;

FIG. 2A is an explanatory diagram schematically showing a manner inwhich an image is projected on a screen by the image forming dec ice:

FIG. 2B illustrates a shape of an image projected on the screen whenobserved from a front of the screen;

FIG. 3A shows of a projection of an image onto a screen with theprojector of FIG. 1;

FIG. 3B shows a trapezoidal image projected onto a surface by theprojection in FIG. 3A;

FIG. 4A is an explanatory diagram showing an overview of digitalsignage;

FIG. 4B illustrates a trapezoidal image projected onto a surface by theprojection in FIG. 4A;

FIG. 5 is a schematic configuration diagram of a HUD system of anautomobile;

FIG. 6 is a schematic configuration diagram of an image display deviceconstituting a HUD system;

FIG. 7 is an explanatory view of a depth of an image projected by animage display device with a short projection distance;

FIG. 8A shows a projection of a trapezoidal image by a projector onto ascreen;

FIG. 8B shows an image forming element of the projector in FIG. 8A;

FIG. 8C shows the trapezoidal image projected onto the screen in FIG.8A;

FIG. 9A shows a projection of a trapezoidal image by an image displaydevice onto a screen;

FIG. 9B shows the trapezoidal image projected onto the screen in FIG.9A;

FIG. 10 is a schematic diagram showing a configuration of an imagedisplay device;

FIG. 11A shows a projection of a trapezoidal image by an image displaydevice onto a screen;

FIG. 11B shows the trapezoidal image projected onto the screen in FIG.11A;

FIG. 12 is a schematic diagram showing a configuration of an imagedisplay device in a HUD;

FIG. 13 is a view for explaining an example of light blocking by a lightshielding unit of a lens barrel in an image display device;

FIG. 14 is a view for explaining an example of light blocking by a lightshielding unit of a lens barrel in an image display device;

FIG. 15 is a view for explaining an example of light blocking by a lightshielding unit of a lens barrel in an image display device;

FIG. 16A is an explanatory view showing an example of an indirect memberprovided in a folding mirror; and

FIG. 16B is another explanatory view showing an example of an indirectmember provided in a folding mirror.

DETAILED DESCRIPTION OF THE DRAWINGS

Hereinafter, an image display device and a HUD system will be describedwith reference to the drawings.

First, a general configuration of a projector including a reflectiveimage forming element will be described with reference to FIG. 1.

FIG. 1 shows an optical engine inside a projector. An image formingelement 30, which may be a DMD, is illuminated with light from lamp 11 aof light source 11, and an enlarged image of the image forming element30 is projected onto a projected surface (hereinafter also referred toas “screen”) by a projection optical system.

Specifically, the light from the lamp 11 a is made into a parallel beamby reflector 11 b and to be incident on fly-eye lens 12 as the lightmixing element.

The incident light is divided into a plurality of light beams by thefly-eye lens (lens array) 12, and is converged again as one light bundleon the image forming surface on the image forming element 30 byillumination lens 61, mirror 62, and curved mirror 63.

By dividing the light source light having unevenness in light quantityinto a plurality of light bundles and then forming one light bundleagain, the illuminance distribution of the image forming element 30 isin a uniform state without uneven light quantity. Since the imageforming element 30 has a uniform illuminance distribution, theilluminance distribution of the image projected on the screen as theenlarged image also becomes uniform.

The image forming element 30 is a device composed of a plurality ofmicromirrors, and the angle of each mirror is changed from, for example,+12° to −12°. For example, when the angle of the mirror is −12°, thereflected light of the illumination light enters the projection lens,and when it is +12, the reflected light of the illumination light is setnot to enter the projection lens. In this way, by controlling the angleof each mirror of the image forming element 30, it is possible to form adigital image on the screen 40.

In the example of the general image display apparatus shown in FIG. 1,the projection optical system includes a refractive optical system 21composed of four lenses.

The refractive optical system 21 is a coaxial optical system sharing theoptical axis of each lens. That is, the light beam reflected by theimage forming element 30 and the optical axis L shared by each lens ofthe refractive optical system are on the same straight line.

In the image display apparatus o described later, a perpendicular lineof the plane including the image forming surface of the image formingelement and the optical axis L of the refractive optical system are noton the same straight line. The optical axis L has an inclination of 1°or more with respect to the perpendicular of the plane including theimage forming surface.

This is similar to a case where the optical path is disposed between theimage forming element and the refractive optical system.

FIG. 2A schematically shows how the image 50 is projected on the screen40 by extracting only the image forming element 30 and the refractiveoptical system 21 of the projector in FIG. 1.

When the image 50 is projected below the height of the table on whichthe projector is installed, the light may be shielded from light.Therefore, the general projector is designed such that the image formingelement 30 is disposed at a position not crossing the optical axis L ofthe refractive optical system 21 (so that it intersects the optical axisL at the plane 30 a including the image forming surface of the imageforming element 30), and the image 50 is projected above the opticalaxis L.

FIG. 2B shows what kind of shape the image 50 formed by the imageforming element 30 is projected on the screen when the screen 40 of FIG.2A is observed from the front. Since both the image forming surface ofthe image forming element 30 and the screen 40 are perpendicular to theoptical axis L, a rectangular image 50 of a similar shape of the imageforming element 30 is naturally projected on the screen 40.

FIG. 3A schematically shows a state in which oblique projection isperformed using the projector of FIG. 1. FIG. 3B shows a trapezoidalimage projected onto a surface by the projection in FIG. 3A.

In FIG. 3A, the image forming element 30 and the refractive opticalsystem 21 are arranged so that the lower end 50 a of the image is infocus, but the focus of the entire image is on the plane perpendicularto the optical axis L (dotted line indicated by reference numeral 51 inFIG. 3A), so that the upper end 50 b of the image is out of focus.

Such unbalance of focus is a significant problem in a projector fordigital signage requiring a short projection distance and a projectorfor a HUD system of an automobile which is required to be able tovisually recognize a displayed image accurately. The outline of thesewill be described below.

FIGS. 4A and 4B illustrate an overview of digital signage. FIG. 4Aschematically shows a state in which the image 50 is projected on thescreen 40 provided on the guide plate 43, and the pedestrian 5 or thelike recognizes this.

Although the image 50 projected on the screen has a trapezoidal shape,it is possible to display the entire trapezoidal shape image shown bythe solid line in FIG. 4B or only the rectangular shape of a part of thetrapezoidal shape image indicated by the dotted line in FIG. 4B.

Since the digital signage can project an image onto an arbitraryprojected surface by the projector, it is possible to use a smallprojector unlike a rear projection television or a general rearprojection projector which requires almost the same size as the screen.Convenience is also excellent on the transport surface.

Projectors for digital signage applications require short projectiondistances. However, as shown in FIG. 7, the shorter the projectiondistance is, the more the unbalance in focus is expanded.

FIG. 7 is a view showing defocus range (the depth of focus) of the image50 projected by the short projection distance projector.

In the lower end 50 b of the image, since light rays are verticallyincident on the screen 40, the image of constant quality (the range inwhich it is in focus) is the distance itself between “♦” and “♦” in FIG.7. On the other hand, at the image upper end 50 a, since light raysdiagonally enter the screen 40, the depth in the screen verticaldirection is a value obtained by multiplying “⋄” and “⋄” by the cosineof the incident angle, as indicated by an arrow 40 b in FIG. 7, isshorter than the lower end arrow 40 b. The depth difference between theimage upper end 50 a and the image lower end 50 b is enlarged as theprojection distance (the distance between the refractive optical system21 and the screen 40 in FIG. 7) is shortened. Therefore, when such aprojector is used for a digital signage application, the focus imbalanceshown in FIG. 3A becomes worse.

FIG. 5 is a schematic configuration diagram of a HUD system of anautomobile. As shown in FIG. 5, the HUD system includes an image displaydevice (small size projector) 1 is disposed and the image projected onthe small screen 41 is projected onto the windshield 3 through anobservation optical system such as a concave mirror 2, so that thedriver 4 can recognize the windshield 3 and recognize it as a virtualimage ahead of it.

It is preferable for the compact projector 1 to project the trapezoidalimage onto the small screen 41, for example, in order to accommodate theprojector in a small enclosure inside the vehicle body, or in order toform an image of a trapezoidal shape opposite to the trapezoidal shapegenerated by the free-form surface shape of the glass 3. In either case,an imbalance in focus of the image is required to be minimal.

In order to eliminate the imbalance of focus in a trapezoidal image, theoptical system may be adjusted so as to satisfy the Scheimpflugcondition. The focus imbalance as described with reference to FIG. 3Acould be resolved by disposing the image forming element 30 of theprojector, as shown in FIG. 4A and FIG. 6, at an angle with respect tothe optical axis L of the refractive optical system 21 constituting theprojection optical system 20.

As shown in FIG. 8A, the screen 40 is tilted with respect to the opticalaxis L of the refractive optical system 21, in the case where atrapezoidal image as shown in FIG. 8C is projected, as shown in FIG. 8Bif the illuminance S1 on the image forming element is uniform, the lightquantity is equal at any position in the trapezoidal shape image.Although, since the illuminance is a value obtained by dividing thelight amount by the area, as shown in FIG. 8C, the illuminance S3 on theshort side 50 b side having a small area is high and the illuminance S2on the long side 50 a side with a large area is low. In this way, it isnecessary to eliminate the imbalance in the illuminance distribution inthe trapezoidal image, but it can be solved by the configurationdescribed below.

FIGS. 9A and 9B schematically illustrate projection of an image by animage display device and FIG. 10 shows the image display device of thepresent application.

As shown in FIG. 9A, the image display apparatus projects an enlargedimage of the image forming element 30, which is an image formingelement, on the screen 40 as a projected surface by the refractiveoptical system 21. As shown in FIG. 9B, the image 50 projected on thescreen 40 has a trapezoidal shape.

In order to eliminate the focus unbalance of the image 50, theperpendicular line to the plane including the image forming surface ofthe image forming element 30 is inclined with respect to the opticalaxis L.

The inclination angle of the image forming element 30 (θ1) can becalculated using the inclination angle of the screen 40 (θ2, theinclination angle of the perpendicular line 40 c to the plane includingthe image forming surface with respect to the optical axis L) and thevalue of the lateral magnification of the refractive optical system 21(M) on the basis of the following equation (1) (Scheimpflug conditionformula):tan(θ2)=(M)×tan(θ1)  (1)When the inclination angle of the screen with respect to the opticalaxis L is 30° (=θ2) and the lateral magnification of the refractiveoptical system 21 is 30 times (=M), the obtained value is 1.1° (=θ1).

That is, by setting the inclination angle of the image forming element30 (the inclination angle of the perpendicular line to the planeincluding the image forming surface with respect to the optical axis L)to 1.1°, the imbalance of focus is eliminated.

In FIGS. 9A and 10, the inclination of the image forming element 30 isexaggerated.

As shown in FIG. 10, the image display device includes a light source11, an image forming element 30 in which a plurality of micromirrors arearranged, an illumination optical system 1) for guiding the lightemitted from the light source 11 to the image forming element 30, and aprojection optical system 20 having a refractive optical system 21composed of a plurality of lenses sharing an optical axis L forprojecting an image formed by the image forming element 30 onto aprojected surface.

The optical axis of the refractive optical system 21 has an inclinationof 1° or more with respect to the perpendicular line to the planeincluding the image forming surface of the image forming element 30 andthe image projected on the projected surface is in a trapezoidal shape

In the illumination optical system 10, the light from the light source11 is incident on a fly-eye lens 12 which is an optical mixing element.

The incident light is divided into a plurality of light bundles by afly-eye lens (lens array) 12, and then converged as one light bundleagain on the image formation surface on the image forming element 30 viaa condensing optical system.

The illumination optical system 10 has a condenser lens 13 and a fieldlens 15 as a condensing optical system and is disposed in this orderfrom the light source 11 side.

The focal point of the focusing optical system (the point where raysconverge when the parallel light is incident on this light collectingoptical system) is a position shifted toward the field Ions 15 side fromthe image forming surface of the image forming element 30. The amount ofshift of the focal point 16 toward the field lens 15 side is small onthe side where the long side of the trapezoid shape is projected whenprojected on the projected surface and large on the side on the shortside when projected on the projected surface.

That is, the focal point as rays converge onto image forming element 30from field lens 15, a first focal point of the rays on a first side ofimage forming element 30 is further from image forming element 30 than asecond focal point of the rays on a second side of image forming element30. Rays with the first focal point, on the first side of image formingelement 30, interact with image forming element 30 and form a small sideof the trapezoid shape. Rays with the second focal point, on the secondside of the image forming element 30, interact with image formingelement 30 and form a long side of the trapezoid shape. Thus, an amountof shift of a focal point of rays on the first side of image formingelement 30 is large while an amount of shift of a focal point of rays onthe second side of image forming element 30 is small.

In addition to the refractive optical system 21, the projection opticalsystem 20 may include a total reflection prism, a bending mirror, andthe like.

As shown in FIG. 10, in the aspect in which the projection opticalsystem 20 has the total reflection prism 22 between the image formingelement 30 and the refractive optical system 21, the optical axis L ofthe refractive optical system 21 which is bent by 90° by the totalreflection prism 22 and enters the image forming element 30 has aninclination of 1° or more with respect to the perpendicular line to theplane including the image forming surface of the image forming element30.

In the image display apparatus, the focal point 16 of the condensingoptical system is set near the side close to the refractive opticalsystem 21 of the image forming element 30. Consequently, the focusingoptical system is in focus on the side close to the refractive opticalsystem 21 of the image forming element 30, the out of focus state is atthe side far from the side, the illuminance on the near side is high,and the illuminance on the far side is low. In this way, illuminanceimbalance of the projected trapezoidal shape image can be eliminated.

In addition, the condenser lens 13 and the field lens 15 of theillumination optical system are preferably lenses having positiverefractive power. As a result, the curvature of field at the focal point16 can be maximized, so that blurring on the short side of thetrapezoidal shape image is induced on the image forming element 30, theilluminance is lowered, and the illuminance distribution of the imageprojected on the screen 40 the imbalance can be neutralized.

Furthermore, by preventing the folding mirror 14 and the totalreflection prism 22 from having refractive power, the field curvature atthe focal point 16 is set in a direction in which the image side of theimage forming element 30 is in focus with respect to the light sourceside, and the image forming element 30 On the surface farther from thedioptric system, more out of focus will proceed and illuminance loweringdue to the low degree of convergence of light can be generated.

Here, in the aspect of image projection by the image display deviceshown in FIG. 9A, a case where the distance to the screen 40 is furthershortened and the lateral magnification of the refractive optical system21 is reduced to 20 times will be described.

In this case, if the inclination angle of the image forming element 30(the inclination angle with respect to the optical axis L of theperpendicular line to the plane including the image forming surface) foreliminating the focus imbalance is calculated based on theabove-described formula 1 (Scheimpflug conditional expression), when theinclination of the screen normal line 40 c with respect to the opticalaxis L is 30° and the lateral magnification of the refractive opticalsystem 21 is 20 times, the obtained value is 1.70.

That is, by setting the inclination angle of the image forming element30 (the inclination angle of the perpendicular to the plane includingthe image formation surface with respect to the optical axis L) of 1.7°,the imbalance in focus is eliminated.

As the inclination angle is changed, the positional relationship betweenthe focal point 16 of the illumination optical system and the imageforming surface of the image forming element 30 also changes. Since thefulcrum supporting the tilt of the image forming element 30 is the pointnearest to the optical axis L, the point where the tilt of the imageforming element 30 moves the greatest is the point illuminated so thatthe illuminance becomes the highest. Therefore, the illuminancedistribution also changes remarkably.

Therefore, the image display apparatus is provided with a folding mirror14 that can change the angle of the light ray incident on the imageforming element 30 according to the inclination of the image formingelement 30 as a light converging optical system.

By changing the inclination of the folding mirror 14, it is possible tomove the focal point 16 of the illumination optical system in thevertical direction in FIG. 10, and it is possible to adjust so that adesired illuminance distribution can be obtained on the image formingelement 30.

It is preferable that the tilt angle of the return mirror 14 is adjustednot by the structure supporting the same, but by separately arranging anindirect member and changing the configuration of the indirect member.

FIG. 16A shows a state where the folding mirror 14 is held by theholding member 14 a schematically. The holding member 14 a is notinvolved in changing the inclination angle, and the inclination angle ischanged by the indirect member 14 b shown in FIG. 16B.

The indirect member 14 b is a thin plate like member arranged so as tobe sandwiched between the reflecting surface of the folding mirror 14and the holding member 14 a.

By appropriately adjusting the thickness of the indirect member 14 b(for example, by appropriately selecting from the plurality of indirectmembers 14 b having different thicknesses), it is possible to adjust theturning mirror 14 so as to have a desired inclination angle.

FIGS. 11A and 11B illustrate projection of an image by an image displaydevice. As shown in FIGS. 11A and 11B, the image display apparatusfurther includes a folding mirror 17 that bends the light from therefractive optical system 21 and leads the light to the screen 40 as aprojected surface.

The reflectance of the folding mirror 17 is such that the reflectance ofthe principal ray reaching the short side of the trapezoidal imageprojected on the screen 40 is smaller than the reflectance of the mainray reaching the long side.

Here, the main light rays reaching the short side of the trapezoidalshape image 50 projected on the screen 40 and the main light raysreaching the long side mean light rays that have passed through theaperture center of the aperture stop of the lens barrel.

The aperture diaphragm of the lens barrel is one of the light shieldingmeans provided on the lens barrel which accommodates and holds theplurality of lenses of the refractive optical system 21 and regulatesthe brightness of the optical axis L of the refractive optical system21. The light beam that has passed through the center of the aperturediaphragm means a light beam that can pass when the aperture stop ismade to a small aperture diameter to an extreme.

As the folding mirror 17, for example, it is preferable that anantireflection coating having reflectance different for each incidentangle of light is set on the aluminum mirror surface.

Generally, as the angle of incidence of light on the mirror increases,the reflectance decreases. However, in the folding mirror 17 of theimage display apparatus, on the contrary, as the incident angle of lightto the mirror is larger, the reflectance may be set to be lower as thelight ray incidence angle is smaller.

In the folding mirror 17, the imbalance in the illuminance distributionof the trapezoidal shape image can be eliminated by making thereflectance of the light ray going to the short side smaller than thereflectance of the light ray going to the long side.

The folding mirror 17 is not a large sized mirror close to a screen sizelike a rear projection television, but a small mirror arranged in thevicinity of the refractive optical system 21.

The image display device may further comprise a small projector (virtualimage observation device) constituting a HUD system for automobiles, asillustrated in FIGS. 5 and 12.

Such a head-up display system includes the image display device, and anobservation imaging optical system for forming a virtual image based onan image formed on the image forming element 30 of the image displaydevice.

The observation optical system has a concave mirror for magnifying andprojecting an image toward a transmissive reflective member(windshield). A schematic configuration of the HUD system is shown inFIG. 5.

As described above, in the HUD system shown in FIG. 5, an image displaydevice (compact projector) 1 is disposed in a vehicle and an image of atrapezoid shape projected on the small screen 41 is projected onto anobservation light such as a concave mirror 2 by projecting it on thewindshield 3 through the observation optical system such as the concavemirror 2 or the like, the driver 4 can recognized it as a virtual imageforward of the windshield 3 with respect to the driver 4 as a reference.

FIG. 6 shows a schematic configuration of the small projector 1, andFIG. 12 shows the configuration of the optical system. The compactprojector 1 includes a light source (see FIG. 12), an image formingelement 30, a projection optical system 20, and a small screen 41. Atrapezoidal image is projected on the small screen 41. As the lightsource 11 of the image display device arranged in the automobile, alaser or an LED having a long life is preferable.

As shown in FIG. 12, the light source includes LEDs of three colors ofred (11R), green (11G), and blue (11B), and the emitted divergent lightis collimated by the coupling lens 18 and converts them into beams,which are combined by a dichroic mirror 19.

Since the space in which the equipment can be placed is limited withinan automobile, space saving is required. In the image display device,the image forming element 30 is arranged so that the image formingsurface is in a direction substantially parallel to the refractiveoptical system 21. In such an arrangement, the reflection on theinclined surface of the total reflection prism 22 is performed in theoptical path from the image forming element 30 to being incident on therefractive optical system 21.

Therefore, the optical axis L of the refractive optical system 21 isbent by 90° in the total reflection prism 22, and then enters the imageforming element 30. In such a case, it is possible to specify thepositional relationship between the short side and the long side of thetrapezoidal shape image with reference to the optical axis L. Theoptical axis L passes through the center of the image forming element 30and also passes the center of the screen 40.

Next, light shielding inside the refractive optical system 21 of FIG. 12will be described. The refractive optical system 21 includes a lensbarrel 25 that houses and holds a plurality of lenses. The lens barrel25 includes light shielding means for shielding light rays which are notprojected onto the projected surface 40 among light rays incident on therefractive optical system 21 from the image forming element 30.

The light shielding means may include an aperture stop 24 for definingthe brightness of the optical axis L of the refractive optical system 21and a marginal ray shielding diaphragm 23 which shields at least a partof the light rays emitted from the region excluding the point where theoptical axis L intersects the perpendicular line to the plane includingthe image formation surface of the image forming element 30.

In FIG. 12, the light emitted from the image forming element 30 isreflected by the inclined surface of the total reflection prism 22 andthen enters the refractive optical system 21. A region (a distance fromthe optical axis in the direction orthogonal to the optical axis) wherethe light beam passes through each lens in the refractive optical system21 is limited by the aperture stop 24 and the marginal ray shieldingdiaphragm 23. By shielding a part of light rays traveling toward theshorter side of the trapezoidal image inside the refractive opticalsystem 21 against the incident light and lowering the light amount onthe short side, it is possible to eliminate the imbalance in theilluminance distribution.

The marginal ray shielding diaphragm 23 is disposed at least between theaperture diaphragm 24 and the image forming element 30 and it ispreferable that the light quantity of the light beam shielded by themarginal ray shielding diaphragm 23 arranged between the aperturediaphragm 24 and the image forming element 30 is larger than the lightquantity of the light beam shielding by another marginal ray shieldingdiaphragm 23 disposed between the aperture stop 24 and the screen 40.

Light shielding by the marginal ray shielding diaphragm 23 disposedbetween the aperture diaphragm 24 and the image forming element 30 willbe described with reference to FIGS. 13-15. In each of FIGS. 13-15, atotal reflection prism 22 is omitted.

A ray bundle (shown as three rays) shown in FIG. 13 is a ray bundleemitted from a point where the optical axis L of the refractive opticalsystem 21 extends and crosses the image forming element 30. And itsdiameter is limited by the aperture stop 24. This light beam diameter isa basic amount that determines the brightness of the refractive opticalsystem 21 and is called “numerical aperture” or “F value”.

The ray bundle (shown as four light rays) shown in FIG. 14 is a raybundle incident on the refractive optical system 21 from a pointdeviated from the optical axis L on the image forming element 30. A partof this ray bundle is shielded by the marginal ray shielding diaphragm23 before reaching the aperture diaphragm 24. Therefore, the ray bundledoes not pass through the entire aperture diameter of the aperture stop24, and the amount of light reaching the screen is smaller than that inFIG. 13.

By utilizing the function of the marginal ray shielding diaphragm 23inside the refractive optical system 21, the light beam bundle on theshort side of the trapezoidal shape image is positively shielded betweenthe aperture diaphragm 24 and the image forming element 30 as shown inFIG. 14, it is possible to eliminate the illuminance imbalance.

Furthermore, as shown in FIG. 15, by tilting the light beam entering therefractive optical system 21 from the image forming element 30 towardthe long side of the trapezoidal image projected on the screen 40 withrespect to the optical axis L of the refractive optical system 21, thelight shielding effect described above is further improved.

In FIG. 15, the symbol “▴” is attached to the light ray directed to thelonger side of the trapezoidal shape image, and the symbol “●” to thelight ray going to the shorter side. As is apparent from FIG. 15, sincethe amount of light shielded by the marginal ray shielding diaphragm 23is increased by the light rays toward the shorter side, the imbalance inthe illuminance distribution of the trapezoidal image is eliminated.

In the refractive optical s, stem 21 shown in each of FIGS. 12-15,disposing the marginal ray shielding diaphragm 23 between the aperturestop 24 and the screen 40, on the contrary, shields the light on theshort side of the trapezoidal image. Therefore, it is preferable thatthe diaphragm disposed between the aperture stop 24 and the screen 40has a minimum light-shielding performance such as for flare removal.

With such a configuration, for example, when the refractive opticalsystem 21 is detached from the apparatus, the ray bundle emitted fromthe image forming element 30 is directed toward the long side of thetrapezoidal image on the screen 40.

In addition, when comparing the amount of light emitted from one pixelof the image forming element 30 rather than the illuminance obtained bydividing the light amount by the screen area, the light amount of thelight beam bundle heading toward the longer side of the trapezoidalimage is larger than the light amount of the light beam bundle directedtoward the short side of the trapezoidal image.

As described above, in the image display device of the presentapplication, the focal point 16 of the condensing optical system is setat a position closer to the point corresponding to the long side of thetrapezoidal shape image than the center of the image forming element 30,and by making the condenser lens 13 and the field lens 15 lenses havingpositive refractive power, it is possible that the illuminance of thepoint corresponding to the long side of the trapezoidal image on theimage forming element 30 is made higher than the illuminance of thepoint corresponding to the short side.

Further, depending on the angle of the folding mirror 14, it is possibleto set the angle of the ray bundle reflected by the image formingelement 30 toward the refractive optical system 21 (The angle towardwhich it is reflected after being reflected by the total reflectionprism 22) to be inclined toward the long side of the trapezoidal shapewith respect to the optical axis L and due to the inclination and theeffect of the marginal ray shielding diaphragm 23 inside the refractingoptical system 21, it is possible to bias the amount of light toward theshort side and the long side of the trapezoidal image. As a result, itis possible to eliminate the imbalance in the illuminance distributioncaused by the trapezoidal shape of the projected image.

The above descriptions an image display apparatus and device are justexamples, and various modifications, replacements, or combinations canbe made without departing from the scope of the present disclosure bypersons skilled in the art.

What is claimed is:
 1. An image display apparatus, comprising: a lightsource; an image forming element on which a plurality of micromirrorsare arranged; an illumination optical system for guiding light emittedfrom the light source to the image forming element; and a projectionoptical system having a refractive optical system including a pluralityof lenses sharing an optical axis and projecting an image formed by theimage forming element onto a projected surface, wherein the optical axisof the refractive optical system has an inclination of 1° or more withrespect to a perpendicular of a plane including an image forming surfaceof the image forming element, the image projected on the projectedsurface is a trapezoidal shape that includes a first portion and asecond portion, the first portion is parallel to the second portion andthe second portion is longer than the first portion, the illuminationoptical system includes a condenser lens and a field lens as acondensing optical system, the condenser lens is between the field lensand the light source, focal points of the condensing optical system arecloser to the field lens than to the image forming surface of the imageforming element, first rays of the light have a first focal point andinteract with a first side of the image forming element, second rays ofthe light have a second focal point and interact with a second side ofthe image forming element, at the image forming surface of the imageforming element, the first rays are more out of focus than the secondrays, and the first rays form the first portion of the trapezoidal shapeand the second rays form the second portion of the trapezoidal shape. 2.The image display apparatus according to claim 1, wherein a first amountof shift of the first focal point is greater than a second amount ofshift of the second focal point.
 3. The image display apparatusaccording to claim 1, wherein the projection optical system includes atotal reflection prism between the image forming element and therefractive optical system, and the optical axis of the refractiveoptical system between the total reflection prism and the image formingelement has the inclination of 1° or more.
 4. The image displayapparatus according to claim 1, further comprising a converging opticalsystem including a folding mirror that changes an angle of a light beamincident on the image forming element according to the inclination ofthe image forming element apparatus.
 5. The image display apparatusaccording to claim 1, wherein the refractive optical system comprises alens barrel that houses and holds the plurality of lenses, the lensbarrel includes a light shield member for shielding light rays notprojected on the projected surface, among light rays incident on therefractive optical system from the image forming element, and the lightshield member includes an aperture stop defining the brightness of theoptical axis of the refracting optical system and at least a part of thelight rays emitted from a region excluding a point where the opticalaxis intersects the perpendicular line and a marginal ray shieldingdiaphragm which blocks light.
 6. The image display apparatus accordingto claim 5, wherein the marginal ray shielding diaphragm is disposed atleast between the aperture stop and the image forming element, and anamount of the light beam shielded by the marginal ray shieldingdiaphragm is more than an amount of the light beam shielded by anothermarginal ray shielding diaphragm disposed between the aperture stop andthe projected surface.
 7. The image display apparatus according to claim1, wherein a light beam incident on the refractive optical system fromthe image forming element is inclined to the second portion of the imageof the trapezoidal shape with respect to the optical axis of therefractive optical system.
 8. The image display apparatus according toclaim 1, further comprising: a mirror that bends the light from theoptical system and guides the light to the projected surface, whereinthe mirror bends the light such that a reflectance of the first raysreaching the first portion of the image of the trapezoidal shape issmaller than a reflectance of the second rays reaching the secondportion of the image of the trapezoidal shape.
 9. The image displayapparatus according to claim 8, wherein the first rays and the secondrays have passed through the aperture center of an aperture stop of thelens barrel.
 10. The image display apparatus according to claim 1,wherein the condenser lens and the field lens of the illuminationoptical system are lenses having positive refractive power.
 11. Ahead-up display system, comprising: an image forming element; anillumination optical system that guides light emitted from a lightsource to the image forming element; a plurality of lenses that projecta trapezoidal image formed by the image forming element; and anobservation imaging optical system for forming a virtual image based onthe trapezoidal image, wherein the observation optical system includes aconcave mirror for enlarging and projecting the trapezoidal image towarda transmissive reflective member, the plurality of lenses have anoptical axis that is at an inclination of 1° or more with respect to aperpendicular of a plane including an image forming surface of the imageforming element, the trapezoidal image includes a first portion and asecond portion, the first portion is parallel to the second portion andthe second portion is longer than the first portion, the illuminationoptical system includes a condenser lens and a field lens, the condenserlens is between the field lens and the light source, first rays of thelight have a first focal point and interact with a first side of theimage forming element, second rays of the light have a second focalpoint and interact with a second side of the image forming element, atthe image forming surface of the image forming element, the first raysare more out of focus than the second rays, and the first rays form thefirst portion of the trapezoidal shape and the second rays form thesecond portion of the trapezoidal shape.
 12. The head-up display systemaccording to claim 11, wherein a first amount of shift of the firstfocal point is greater than a second amount of shift of the second focalpoint.
 13. The head-up display system according to claim 11, furthercomprising a projection optical system includes a total reflection prismbetween the image forming element and a refractive optical system, andthe optical axis of the refractive optical system between the totalreflection prism and the image forming element has the inclination of 1°or more.
 14. The head-up display system according to claim 11, furthercomprising a converging optical system including a folding mirror thatchanges an angle of a light beam incident on the image forming elementaccording to the inclination of the image forming element apparatus. 15.The head-up display system according to claim 11, wherein a refractiveoptical system comprising a lens barrel that houses and holds aplurality of lenses, the lens barrel includes a light shield member forshielding light rays not projected on a projected surface, among lightrays incident on the refractive optical system from the image formingelement, and the light shield member includes an aperture stop definingthe brightness of the optical axis of the refracting optical system andat least a part of the light rays emitted from a region excluding apoint where the optical axis intersects the perpendicular line and amarginal ray shielding diaphragm which blocks light.
 16. The head-updisplay system according to claim 15, wherein the marginal ray shieldingdiaphragm is disposed at least between the aperture stop and the imageforming element, and an amount of the light beam shielded by themarginal ray shielding diaphragm is more than an amount of the lightbeam shielded by another marginal ray shielding diaphragm disposedbetween the aperture stop and the projected surface.
 17. The head-updisplay system according to claim 15, wherein a light beam incident onthe refractive optical system from the image forming element is inclinedto the second portion of the image of the trapezoidal shape with respectto the optical axis of the refractive optical system.
 18. The head-updisplay system according to claim 11, further comprising: a mirror thatbends the light from the optical system and guides the light to aprojected surface, wherein the mirror bends the light such that areflectance of the first rays reaching the first portion of the image ofthe trapezoidal shape is smaller than a reflectance of the second raysreaching the second portion of the image of the trapezoidal shape. 19.The head-up display system according to claim 18, wherein the first raysand the second rays have passed through the aperture center of anaperture stop of the lens barrel.
 20. An image display apparatus,comprising: an image forming element; an illumination optical systemthat guides light emitted from a light source to the image formingelement; and a plurality of lenses that project a trapezoidal imageformed by the image forming element onto a projected surface, whereinthe plurality of lenses have an optical axis that is at an inclinationof 1° or more with respect to a perpendicular of a plane including animage forming surface of the image forming element, the trapezoidalimage includes a first portion and a second portion, the first portionis parallel to the second portion and the second portion is longer thanthe first portion, the illumination optical system includes a condenserlens and a field lens, the condenser lens is between the field lens andthe light source, first rays of the light have a first focal point andinteract with a first side of the image forming element, second rays ofthe light have a second focal point and interact with a second side ofthe image forming element, at the image forming surface of the imageforming element, the first rays are more out of focus than the secondrays, and the first rays form the first portion of the trapezoidal shapeand the second rays form the second portion of the trapezoidal shape.